The present invention relates to a molecular single-crystalline film (which herein refers to a film having a thickness of at most ca. 100 xcexcm and having a portion which retains a single crystal state having a uniform molecular crystalline alignment over the thickness and over an areal extension including a side length of at least 10 times the thickness, i.e., an areal size useful as a functional film, preferably an areal size of at least 50 xcexcmxc3x9750 xcexcm) and a process for the production thereof.
A molecular crystal can be expected to be a useful device material, such as a superconducting material, an effective photoconductor or a gas sensor, because of its electrical and geometrical structure and packing state. As the process for the production thereof, growth in a solution and growth in a molten state have been generally practiced. According to any of such processes, however, it is difficult to obtain a thin film of single crystal by suppressing an increase in thickness, and this poses an obstacle against using it as a functional layer in devices which have a laminar structure in many cases. As another process, there is known a gas phase deposition process, by which, however, it is difficult to prepare a uniform film due to affection by the gas phase deposition boundary.
On the other hand, carrier transportation performance has been reportedly improved by utilizing a molecular alignment in a higher order liquid crystal phase of SmB or SmE (Ohyou Butsuri, Appl. Phys., vol. 68, no. 1, pp. 26-32 (1999)). In this report, a higher speed transportation of electrons and holes has been aimed at by utilization of alignment order in a higher order liquid crystal phase. The improvement in high speed transportation performance has been considered attributable to the formation of flow paths for electrons and holes due to regular packing of aromatic rings in the higher order smectic phase alignment. This performance has been also noted as a carrier transportation layer in EL devices, and further improvement is expected.
Regardless of whether it is a liquid crystal or a (solid) crystal (herein a term xe2x80x9ccrystalxe2x80x9d without further notation is used to mean a solid crystal), the film thereof is required to assume a single crystal state free from defects (i.e., free from carrier traps) in order to function as a functional layer as mentioned above.
Then, if a (solid) single-crystalline film can be obtained, it is expected to achieve a higher speed and higher density carrier transportation because of a higher degree of order and a closer packing of molecules than a liquid crystal film.
In view of the above-mentioned circumstances, a principal object of the present invention is to provide a molecular single-crystalline film useful in a device and a process for the effective production thereof.
In order to achieve the above-mentioned object, it may be conceived to form a liquid crystal material layer of which the thickness is regulated between a pair of boundaries at a higher temperature and cool the liquid crystal material layer to room temperature, thereby forming a crystal layer wherein the molecular alignment is fixed. As a result of my study, however, such a crystal film obtained through the above-described process, in general, can only form a polycrystalline film and fails to provide a single-crystalline film. This is believed to be due to a strain or disclination in a domain relaxed in a liquid crystal phase because of fluidity or flexibility of the liquid crystal phase, which causes precipitation of crystallites or polycrystallization during crystallization.
As a result of my further study, it has been discovered that a liquid crystal layer of a certain class of liquid crystal having a better regularity formed in a thickness regulated by a pair of boundaries can be phase-transformed into a single-crystalline film while remarkably suppressing polycrystallization.
Based on the above discovery, according to a first aspect of the present invention, there is provided a single-crystalline film having a molecular alignment order provided through phase transition from a liquid crystal phase.
I have also discovered a smectic liquid crystal material exhibiting a uniform (i.e., a single mode of) molecular alignment inclusive of a director (i.e., molecular long-axis) direction in a smectic layer as a suitable material as the above-mentioned liquid crystal material having a better regularity.
Based on the discovery, according to a second aspect of the present invention, there is provided a process for producing a single-crystalline film comprising a step of disposing a smectic liquid crystal material exhibiting a uniform molecular alignment in a smectic layer between a pair of boundaries having a thickness regulation function, and a crystallization step of cooling and solidifying the smectic liquid crystal material through its smectic phase into a single-crystalline film.
A suitable example of such a smectic liquid crystal material is one having a molecular structure, which is symmetrical with respect to its molecular long axis direction. The molecular structure of such a smectic liquid crystal may be represented as a so-called headxe2x80x94head structure, and the mode of molecular lamination alignment thereof in a smectic layer is only one, i.e., cannot be other than stacking of headxe2x80x94head molecules, so that it does not readily result in crystal defects at the time of phase transition into the crystal. In contrast thereto, while many higher temperature smectic liquid crystal materials, i.e., liquid crystal materials having a smectic phase at an elevated temperature, have been known, most of them have a molecular structure which is asymmetrical with respect to the molecular long axis direction and may be represented as a so-called head-tail structure. It is considered that such molecules are stacked in a random manner, inclusive of head-tail, tail-head, . . . , to form a smectic layer, so that many crystal defects are liable to occur at the time of phase transition into crystal, thus providing a polycrystalline film.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.